package dotty.tools
package dotc
package core
package tasty

import util.Util.dble

object TastyBuffer {

  /** The number of digits of the natural number `nat`, written in base 128 format. */
  def natSize(nat: Int): Int =
    if (nat < 128) 1 else natSize(nat >>> 7) + 1

  /** An address pointing to an index in a Tasty buffer's byte array */
  case class Addr(index: Int) extends AnyVal {
    def - (delta: Int): Addr = Addr(this.index - delta)
    def + (delta: Int): Addr = Addr(this.index + delta)

    def relativeTo(base: Addr): Addr = this - base.index - AddrWidth
  }

  val NoAddr = Addr(-1)

  /** The maximal number of address bytes.
   *  Since addresses are written as base-128 natural numbers,
   *  the value of 4 gives a maximal array size of 256M.
   */
  final val AddrWidth = 4

  /** An address referring to a serialized name */
  case class NameRef(index: Int) extends AnyVal
}
import TastyBuffer._

/** A byte array buffer that can be filled with bytes or natural numbers in TASTY format,
 *  and that supports reading and patching addresses represented as natural numbers.
 */
class TastyBuffer(initialSize: Int) {

  /** The current byte array, will be expanded as needed */
  var bytes = new Array[Byte](initialSize)

  /** The number of bytes written */
  var length = 0

  // -- Output routines --------------------------------------------

  /** Write a byte of data. */
  def writeByte(b: Int): Unit = {
    if (length >= bytes.length)
      bytes = dble(bytes)
    bytes(length) = b.toByte
    length += 1
  }

  /** Write the first `n` bytes of `data`. */
  def writeBytes(data: Array[Byte], n: Int): Unit = {
    while (bytes.length < length + n) bytes = dble(bytes)
    Array.copy(data, 0, bytes, length, n)
    length += n
  }

  /** Write a natural number in big endian format, base 128.
   *  All but the last digits have bit 0x80 set.
   */
  def writeNat(x: Int): Unit =
    writeLongNat(x.toLong & 0x00000000FFFFFFFFL)

  /** Write a natural number in 2's complement big endian format, base 128.
   *  All but the last digits have bit 0x80 set.
   */
  def writeInt(x: Int): Unit =
    writeLongInt(x)

  /**
   * Like writeNat, but for longs. Note that the
   * binary representation of LongNat is identical to Nat
   * if the long value is in the range Int.MIN_VALUE to
   * Int.MAX_VALUE.
   */
  def writeLongNat(x: Long): Unit = {
    def writePrefix(x: Long): Unit = {
      val y = x >>> 7
      if (y != 0L) writePrefix(y)
      writeByte((x & 0x7f).toInt)
    }
    val y = x >>> 7
    if (y != 0L) writePrefix(y)
    writeByte(((x & 0x7f) | 0x80).toInt)
  }

  /** Like writeInt, but for longs */
  def writeLongInt(x: Long): Unit = {
    def writePrefix(x: Long): Unit = {
      val y = x >> 7
      if (y != 0L - ((x >> 6) & 1)) writePrefix(y)
      writeByte((x & 0x7f).toInt)
    }
    val y = x >> 7
    if (y != 0L - ((x >> 6) & 1)) writePrefix(y)
    writeByte(((x & 0x7f) | 0x80).toInt)
  }

  /** Write an uncompressed Long stored in 8 bytes in big endian format */
  def writeUncompressedLong(x: Long): Unit = {
    var y = x
    val bytes = new Array[Byte](8)
    for (i <- 7 to 0 by -1) {
      bytes(i) = (y & 0xff).toByte
      y = y >>> 8
    }
    writeBytes(bytes, 8)
  }

  // -- Address handling --------------------------------------------

  /** Write natural number `x` right-adjusted in a field of `width` bytes
   *  starting with address `at`.
   */
  def putNat(at: Addr, x: Int, width: Int): Unit = {
    var y = x
    var w = width
    if(at.index + w >= bytes.length)
      bytes = dble(bytes)
    var digit = y & 0x7f | 0x80
    while (w > 0) {
      w -= 1
      bytes(at.index + w) = digit.toByte
      y >>>= 7
      digit = y & 0x7f
    }
    assert(y == 0, s"number $x too large to fit in $width bytes")
  }

  /** The byte at given address */
  def getByte(at: Addr): Int = bytes(at.index)

  /** The natural number at address `at` */
  def getNat(at: Addr): Int = getLongNat(at).toInt

  /** The long natural number at address `at` */
  def getLongNat(at: Addr): Long = {
    var b = 0L
    var x = 0L
    var idx = at.index
    do {
      b = bytes(idx)
      x = (x << 7) | (b & 0x7f)
      idx += 1
    } while ((b & 0x80) == 0)
    x
  }

  /** The address (represented as a natural number) at address `at` */
  def getAddr(at: Addr) = Addr(getNat(at))

  /** The smallest address equal to or following `at` which points to a non-zero byte */
  final def skipZeroes(at: Addr): Addr =
    if (getByte(at) != 0) at else skipZeroes(at + 1)

  /** The address after the natural number found at address `at`. */
  final def skipNat(at: Addr): Addr = {
    val next = at + 1
    if ((getByte(at) & 0x80) != 0) next else skipNat(next)
  }

  /** The address referring to the end of data written so far */
  def currentAddr: Addr = Addr(length)

  /** Reserve `AddrWidth` bytes to write an address into */
  def reserveAddr(): Addr = {
    val result = currentAddr
    length += AddrWidth
    result
  }

  /** Fill reserved space at address `at` with address `target` */
  def fillAddr(at: Addr, target: Addr) =
    putNat(at, target.index, AddrWidth)

  /** Write address without leading zeroes */
  def writeAddr(addr: Addr): Unit = writeNat(addr.index)

  // -- Finalization --------------------------------------------

  /** Hook to be overridden in subclasses.
   *  Perform all actions necessary to assemble the final byte array.
   *  After `assemble` no more output actions to this buffer are permitted.
   */
  def assemble(): Unit = ()
}